So big O means that the first function is less than or equal to the second function up to a constant. There does not exist a constant c such that n^2<=c*n for all n. That's why n^2=/=O(n). Try to formalize this into a definition

oh, thanks grief. i think i have now solved it using your clear explanation. obviously, f(n) has to be less than or equal to to some constant, Z, multiplied by g(n) given that another constant is less then f(n).